The hormone ghrelin, known to promote hunger and fat storage, has been found to enhance exploratory "sniffing" in both animals and humans.
The research, by University of Cincinnati (UC) scientists, suggests that ghrelin may be designed to boost detection of calories in our environment through smell and link those inputs with natural regulation of metabolism and body weight.
Led by Jenny Tong, MD, and Matthias Tschöp, MD, both of UC's endocrinology, diabetes and metabolism division, the study appears in the April 13, 2011, issue of The Journal of Neuroscience, the official journal of the Society for Neuroscience.
"Smell is an integral part of feeding and mammals frequently rely on smell to locate food and discriminate among food sources," says Tong. "Sniffing is the first stage of the smell process and can enhance odor detection and discrimination."
The research team tested both rats and humans. Rats were given ghrelin and monitored for sniff frequency using a video-based behavior analysis system set to record the movement of the nose tip. The investigators also measured the ability of the rats to detect specific odors mixed in water.
Human subjects were evaluated before and after ghrelin infusion using a sniff magnitude test (SMT) developed at the University of Cincinnati by co-investigator Robert Frank, PhD. Subjects were instructed to take a natural sniff of several odorants using the SMT canister and rate the smells in order of pleasantness. Software connected to the canister allowed researchers to measure sniff pressure to determine overall sniff magnitude.
Data for both humans and rats show ghrelin enhanced odor detection and exploratory sniffing.
"Other studies have shown that hunger can enhance odor detection and sniffing in animals," says Tschöp. "Since ghrelin is a hunger-inducing stomach hormone that is secreted when the stomach is empty, this hormone pathway may also be responsible for the hunger-induced enhancement of sniffing and odor detection."
The scientists say this study could open up new avenues connecting metabolic control, chemo-sensation and behavioral neuroscience research. Future studies will explore the exact molecular pathways through which ghrelin affects sniff behavior.
The study was supported by grants from the National Institutes of Health and the Netherlands Organization for Scientific Research.
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